1. Field of the Invention
The present invention relates to control systems for superconducting magnets, and particularly though not exclusively to such systems for promoting the safe operation of magnets utilized for the examination of patients in magnetic resonance imaging (MRI) scanners.
2. Description of the Prior Art
Superconducting magnet systems typically are formed by coils of superconductive material wound on suitable formers. Such systems are known to be vulnerable to so-called quenching, a sudden and irreversible collapse of the magnetic field arising from any event which can cause localized disturbance of the superconductive state, leading to the creation of electrical resistance at the site of the disturbance, and a rapid build-up of localized heating which exacerbates the resistance. It is well known that, in order to reduce the risk of localized, possibly permanent, damage to the magnet coils it is necessary, shortly after the commencement of a quenching event, to distribute the quench as widely as possible through the coils of the system. This is typically done by providing heaters, located adjacent the magnet coils, which are automatically energised in the event of a quench to raise the resistance of the coils generally.
Aside from the risk of irreversible damage to the coils during a quench, such events also give rise to safety concerns, including the need to rapidly vent boiled-off coolant vapor, and the need to contain powerful stray magnetic fields generated by the rapid collapse of the MRI scanning field generated by the quenching coils. The vapor control systems are well known, and are not addressed by this invention. As regards the containment of the powerful stray fields, this is typically implemented by means of strategically placed “quench bands”, which are continuous bands of highly conductive material, placed to surround one or more of the magnet coils, typically on their outer surfaces. During a quench event, such quench bands support large eddy currents designed to cancel the field expansion caused by the eddy currents in the rest of the magnet.
It is to be noted that quench events are not always accidental. There are occasions when it is necessary to deliberately quench the magnetic field in order (for example) to remove the magnetic field rapidly to accommodate health and safety requirements, and references to quenches and quenching herein should accordingly be construed to encompass both deliberately and accidentally induced quench events.
A further difficulty associated with superconductive magnetic coil systems is that the superconducting wire and the material of the former typically have very different mechanical properties; notably in respect of their coefficients of thermal expansion. A typical superconducting wire has a coefficient of expansion dominated by the copper matrix of the wire, whereas the former material may typically be aluminium, steel or some form of composite material with a coefficient of thermal expansion different from that of copper. This mismatch of thermal expansion coefficients can cause the superconducting windings to become loose on the magnet former when the magnet is cooled down from its manufacturing temperature (typically 300K) to its operating temperature (typically 4K). A further change in the relative radial dimensions of the coils with respect to the former will happen due to the Lorentz force on the coils when the magnet is energized, which will typically cause positive (expansive) hoop stress in the magnet coils, again loosening the coils on the former.
It will be appreciated that looseness of fit between the coils and their formers can increase the vulnerability of the magnets to mechanical shock, which may promote quenching. It may also cause the homogeneity of the generated magnetic field to be compromised owing to the coils no longer retaining their design dimensions or position. Such problems are usually addressed by restraining the coils from significant motion relative to the former, either by the provision of coil clamps or by over-binding the coils with one or more layers of a typically non-superconducting wire, usually aluminium, which tends to tighten onto the former during cool down, due to its higher coefficient of thermal expansion compared to that of the superconducting wire. Such an over-binding can also compensate for expansion caused by hoop stress associated with the Lorentz forces in the magnet coils.
The above-recited three problems; namely the dissipation of quench events, the containment of stray fields on rapid collapse of the main field associated with the superconductive coils and the retention of fit of coils to formers are all typically addressed individually.